Additive to improve voltage regulation in a lithium-copper chloride rechargeable cell

ABSTRACT

An electrochemical cell and method for improving voltage regulation of said cell. The cell employs lithium as an anode, lithium tetrachloroaluminate as an electrolyte, sulfur dioxide as a solvent, and high surface area carbon black containing cupric chloride as a cathode. A lithium halide, such as LiCl or LiBr, is employed to improve voltage regulation.

TECHNICAL FIELD OF INVENTION

The present invention involves an electrochemical cell employing lithiumas an anode, lithium tetrachloroaluminate as an electrolyte, sulfurdioxide as a solvent, and a cathode of high surface area carbon blackcontaining cupric chloride. It has been found that voltage regulation ofsuch cells can be enhanced by the addition of a lithium halide to thecathode.

BACKGROUND OF THE INVENTION

Over the past several years, a class of cells commonly known as liquidcathode cells has emerged as the best candidate to provide substantiallyincreased performance over the age-old zinc-carbon, alkaline and silveroxide cells. The liquid cathode cells are distinguished from moreconventional cells in that the active cathode depolarizer is a liquid.The basic elements of the cell are an anode, typically consisting of analkaline or alkaline earth metal, a current collector consisting of ahigh surface area material that is catalytically active in the reductionof the liquid cathode, a suitable separator located between the currentcollector and the anode and mechanically separating the two, and theelectrolyte, which includes the liquid cathode as well as an ionicallyconductive solute dissolved in a liquid solvent. In certain type cells,the solvent performs the additional function of the active cathodedepolarizer.

The liquid cathode cells that are most commonly discussed in theliterature as having the best performance characteristics are thoseusing lithium metal anodes and active cathode depolarizers that areeither oxyhalides, such as thionyl chloride or sulfur dioxide. Sulfurdioxide has also been employed as the cell solvent, particularly insecondary or rechargeable cells of the type which are made the subjectof the present invention.

Cells for contemplation herein employ a lithium anode and a solution ofa highly soluble lithium salt as the electrolyte, such as lithiumtetrachloroaluminate. Sulfur dioxide is used as the solvent, while thecathode is a high surface area carbon black containing cupric or copperII chloride.

Under ideal conditions, the cupric chloride contained within the porouscarbon cathode discharges, displaying a single voltage plateau atapproximately 3.4V. There is, however, a tendency for the cupricchloride to decompose to cuprous chloride via the reaction:

    2 CuCl.sub.2 →2 CuCl+Cl.sub.2

If impurity levels of copper I chloride are high, the cell displaysadditional voltage plateaus on charge and discharge. Typically, theabove-referenced decomposition reaction results in between 2 and 10%cuprous chloride in the cathode, having the effect outlined above.

It is thus an object of the present invention to provide a rechargeableLi-SO₂ cell having a cathode of porous carbon containing CuCl₂ whileminimizing the effects of CuCl contamination in the cathode.

This and further objects will be more fully appreciated when consideringthe following disclosure and appended drawings, wherein

FIGS. 1 and 2 represent graphs of cell voltages during charge anddischarge for the prior art as well as for cells produced according tothe present invention.

SUMMARY OF THE INVENTION

The present invention is based upon the discovery that a lithium basednon-aqueous cell employing liquid sulfur dioxide as the solvent andcupric chloride contained in a porous carbon body as the cathode willdisplay a single voltage plateau if the impurity which normally occursin the cathode as a result of the reduction of Cu⁺² is minimized oreliminated completely. This impurity results from the formation ofcuprous chloride and chlorine from the cupric chloride cathode additive.The present invention is carried out by adding a lithium halide such asLiBr and LiCl to the cell electrolyte. Ideally, these additives areemployed in amounts between approximately 1% to 15% by weight based uponthe weight of cupric chloride in the positive electrode.

DETAILED DESCRIPTION OF THE INVENTION

During the preparation of carbon-cupric chloride mixtures for use as thepositive electrode of a secondary cell, some of the cupric chloridedecomposes via the reaction

    2 CuCl.sub.2 →2 CuCl+Cl.sub.2                       (1)

This results in a significant percentage of copper I chloride in thecathode generally measured to be between approximately 2 to 10% byweight as measured by titration. The effect of this contamination is tointroduce an additional voltage plateau during the recharging process asshown in FIG. 1.

FIG. 1 represents the charge-discharge voltage curve of an AA cell whichconsisted of a 6 mil lithium anode separated from a 23 mil cathodecomprised initially of copper II chloride and carbon. The separator wasa microporous Tefzel material capable of inhibiting the dendritic growthof lithium. Tefzel is a trademark of an ethylene-tetrafluoroethylenecopolymer membrane available from Raychem Corporation. Prior to windingthe components, the positive electrode was heated to 120° C. undervacuum to remove water. The cell was filled with an electrolyteconsisting of lithium tetrachloroaluminate dissolved in liquid sulfurdioxide.

FIG. 2 represents the charge-discharge voltage curve of a similar AAsize cell which contained additional lithium chloride added directly tothe cathode. The lithium chloride weight was approximately 4% of that ofthe copper II chloride initially present. This cathode (or positiveelectrode) was also subjected to the 120° C. drying procedure.

Turing to FIG. 1, the typical EMF of an "AA" cell is shown during chargeand discharge. At A, charging begins and a first plateau is reached atB. Without contamination, the voltage of the cell would remain constantuntil a peak at D is reached. Instead, a secondary plateau isestablished at C which is indicative of CuCl contamination in thecathode. Discharge then takes place which should result in theestablishment of a secondary plateau at F. Instead, an elbow appears atE which again is indicative of cuprous chloride contamination.

These plateaus are associated with the following reaction:

    CuCl+AlCl.sub.4.sup.- →CuCl.sub.2 +AlCl.sub.3 +e.sup.-(2)

It has been discovered that a most convenient way to minimize oreliminate CuCl contamination is to add a lithium halide to the cathode.Ideal lithium halides for this purpose are lithium chloride and lithiumbromide.

Typically, 1% to 15% by weight of the lithium halide is added to thecathode based upon the weight of the cupric chloride with the resultsshown in FIG. 2. In this case, a cathode containing 2.8 gm of copper IIchloride was doped with 0.14 gm of lithium chloride. As noted in FIG. 2,which is again a plot of EMF voltage of an "AA" cell, during thecharge-discharge cycle, a first plateau is established during thecharging of the cell at G. Once fully charged, a spike is achieved at H,at which time discharge occurs at I. Clearly, two distinct plateaus, oneat charge and one at discharge, are evidenced. No secondary plateaus,such as shown in FIG. 1, are evident in producing a cell pursuant to thepresent invention.

Although not intending to be bound by any particular scientific theoryin explaining the mechanism for the present invention, it is noted thatthe addition of a lithium halide does affect the acidity of the cell.The lithium halide reacts with the aluminum chloride (AlCl₃) of equation(2), above, thus forcing the reaction to the right and consuming thecuprous chloride impurity. Aluminum chloride is liberated in the cell byreaction (2), above, in the first recharge of the cell such thatsubsequent cycles occur in a neutral or slightly basic electrolyte.

The consumption of aluminum chloride and the consequent reduction inacidity of the cell enhances the cell's ability to generate chlorine onovercharge. Chlorine generation is oftentimes viewed as beneficial sinceit is transported to the lithium electrode and reacts with any surfacefilm formed thereon to regenerate lithium tetrachloroaluminate andsulfur dioxide, thus preventing excessive film buildup and lithiumisolation. Under circumstances where the acidity of the cell isvariable, the degree of lithium film erosion is also variable. Insituations where several cells are configured in series or in parallel,this can result in variable degrees of overcharge and thus lithium filmremoval, which can in turn result in mismatched capacities and earlybattery failure.

We Claim
 1. In an electrochemical power cell employing lithium as ananode, lithium tetrachloroaluminate as an electrolyte, sulfur dioxide asa solvent, and high surface carbon black containing cupric chloride as acathode, the improvement comprising adding a lithium halide to thecathode.
 2. The electrochemical cell of claim 1 wherein said lithiumhalide is a member selected from the group consisting of LiCl and LiBr.3. The electrochemical cell of claim 1 wherein said lithium halide ispresent in an amount between approximately 1% to 15% by weight basedupon the weight of cupric chloride in the cathode.
 4. A method ofimproving voltage regulation of an electrochemical cell employinglithium as an anode, lithium tetrachloroaluminate as an electrolyte,sulfur dioxide as a solvent, and high surface area carbon blackcontaining cupric chloride as a cathode, said method comprising theaddition of a lithium halide to the cathode.
 5. The method of claim 4wherein said lithium halide is a member selected from the groupconsisting of LiCl and LiBr.
 6. The method of claim 4 wherein saidlithium halide is present in an amount between approximately 1% to 15%by weight based upon the weight of cupric chloride in the cathode.